1. Field of the Invention
The present invention relates to a method for detecting the height of wires bonded between the pads of semiconductor chips and the leads of lead frames.
2. Prior Art
The method disclosed in U.S. Pat. No. 5,347,362 which corresponds to Japanese Patent Application Laid-Open (Kokai) No. 5-160233 is an example of a conventional method for detecting the height of bonded wires. FIG. 5 shows an apparatus in which this method is used.
In this prior art method, a workpiece 6 in which wires 5 are bonded so as to connect the pads 2 of a semiconductor chip 1 and the leads 4 of a lead frame 3 is placed on a detection stand 10. An illuminating means 11 is positioned above the workpiece 6 which is placed on the detection stand 10. The illuminating means 11 is mounted to the lower part of an optical system 12, which includes lenses and prisms, and an imaging device 14, which is, for example, a CCD (photoelectric transducer element) camera, is mounted to the upper part of the optical system 12 with a diaphragm means 13 in between. The optical system 12 to which the illuminating means 11 and the imaging device 14 are mounted is disposed on a supporting block 16 which is fastened to the surface of an XY table 15. The optical system 12 is moveable up and down by a Z-axis motor 17.
Although not shown in FIG. 5, the illuminating means 11 includes a high-angle illuminating device and a low-angle illuminating device. In the high-angle illuminating device, LED's are disposed in the form of a ring; and in the low-angle illuminating device, LED's are likewise disposed in the form of a ring so that the LED's of the low-angle illuminating device surround the LED's of the high-angle illuminating device. The respective LED's of the high-angle illuminating device and low-angle illuminating device are oriented toward the axial center of the optical system 12. Furthermore, the angle of inclination of the high-angle illuminating device relative to a horizontal plane is set to be approximately 30 to 55 degrees, and the angle of inclination of the low-angle illuminating device relative to the horizontal plane is approximately 5 to 15 degrees.
When the height of a bonded wire is detected, the low-angle illuminating device of the illuminating means 11 is controlled so that, as disclosed in the above-identified prior art, some of the LED's are switched off; and the detection is performed with the focal depth set at a shallow depth using the diaphragm means 13.
More specifically, the XY table 15 shown in FIG. 5 is driven so that the optical system 12 is positioned, as seen in FIG. 6(a), on a vertical line A which passes through the XY coordinates of the wire 5 that is to be detected. Then, the Z-axis motor 17 is driven so that the optical system 12 is raised or lowered, and focusing of the imaging device 14 is made on the detection point A0, thus obtaining the image of the wire 5. The obtained image appears as shown in FIG. 6(b). In FIG. 6(b), the width H0 of the wire appears to be a minimum at the focal point A0 (detection point A0), and the wire width appears to become larger and less distinct as the distance from the focal point A0 increases. As disclosed in the prior art as well, there is a particular relationship between the blurring width (wire width) and the wire height. Accordingly, by knowing this relationship beforehand, it is possible to calculate the height at an arbitrary point of the wire from the obtained wire width.
In the prior art described above, the height of a bonded wire is measured based upon the imaged wire width using the in-focus wire image as a standard. However, there is no disclosure in this prior art concerning the method used to evaluate (i.e., ascertain the acceptability of) the measurement results.